Electric discharge machining (EDM) uses electrical discharges to removal material from the workpiece. As result of this, some material is removed from the tool electrode. This is termed as ‘electrode wear’ in EDM. Electrode wear is a serious drawback of the EDM process. Many studies have been conducted till now, to study the electrode wear phenomenon. The electrode wear depends on a number of factors, such as the size and shape of the tool electrode, the material of the workpiece to be processed, the surface roughness of the workpiece, and so on, which are associated with discharging parameters programmed in the EDM process, like discharging waveform, voltage, servo parameters, etc.
The drilling of holes ranging in diameter from 0.08-3.00 mm without any burrs and free of tapers is largely the domain of electric discharge machining (EDM). Typically, these holes are drilled using a hollow electrode for supplying fluid to the bottom of such micro hole so as to wash out debris and thus prevent shortage. It is known that a fast removal rate can be obtained when negative polarity EDM machining is used with an EDM process, that is, tool electrode is cathode and workpiece is anode. However, the electrode wear of the negative polarity EDM machining is comparatively larger. In a negative polarity EDM machining as a uniform workpiece is processed under the same electric discharging conditions, its electrode wear is almost a constant per unit length of machining amount, in addition, the electrode is increasing with the smaller the diameter of the tool electrode is or the deeper the machining depth of a hole is. Thus, it is important to compensate the electrode wear.
In a method for compensating electrode wear of micro EDM disclosed in TW Pat. No. 90108713, the electrode wear is assumed to be a dynamic value that can be detected and measured in an automatic manner. In brief, an exact electrode wear of a hole, referring as first hole, is obtained by subtracting a tool electrode value detected prior to the machining of a second hole to be machined after the first hole with relation to the machined surface of a workpiece from a reference tool electrode value which is substantially being the tool electrode value detected prior to the machining of the first hole with relation to the machined surface of the same workpiece. After the exact electrode wear of the first is obtained, the obtained exact electrode wear of the first hole is used as a predicted wear of the tool electrode for machining the second hole to a desire depth, that is, the exact electrode wear of the first hole is used as a the electrode wear of the second hole for machining to desire depth. It is assume that the electrode wear of any two successive cavities should be almost the same so that the actual depth of the machined second cavity can be compensated using the exact electrode wear of the first hole with minimum error. However, the aforesaid compensation method can only be applied to those holes machined by only a single tool electrode. When a originally used electrode is exhausted and another new tool electrode is replaced in a hole machining, the aforesaid method will perform no corresponding electrode wear detection and thus there will be no compensation so that machining depth error can not be prevented.
Therefore, in applications of EDM, the electrode wear must be considered while configuring a designed depth of cut so as to enable the actual depth to equal the designed depth. However, the aforesaid method can only work for machining holes using only a single tool electrode and will cause more error when the tool electrode is exhausted and replaced by another new tool electrode, since it can not provide a precise electrode wear of the exhausted electrode tool for the new tool electrode.